1. Field of the Invention
The present invention relates to a system for maximizing the utilization of a display memory used in an electric appliance for office automation such as, for example, a word-processor or a work station. It particularly relates to an electric appliance for office automation of a type utilizing a high resolution display device.
2. Description of the Prior Art
An example of an electric appliance for office automation utilizing a display memory, such an electric appliance for office automation, is shown in FIG. 5 and is currently available in the market. The illustrated electric appliance comprises a graphic central processing unit (CPU) 1, a system memory 2 for storing programs for the graphic CPU 1 and data, a character generator read-only memory (CGROM) 3 for storing character patterns of dot fonts, and a display memory 5 having parallel input/output ports, all of them being connected together by means of a local bus 9. A monochromatic cathode ray tube (CRT) 6, forming a display unit, is connected with the serial output ports of the display memory 5. The prior art system is so designed that pixel data corresponding to each picture to be displayed on the cathode ray tube 6 is, after having been once stored in the display memory 5, outputted to the cathode ray tube 6 according to a raster scanning scheme.
The display memory 5 used in the prior art electric appliance for office automation is a frame buffer compatible to the number of bits per pixel of the cathode ray tube 6 and the screen size of the cathode ray tube 6. Therefore, the display memory 5 has to be designed exclusively for use with the particular cathode ray tube 6. For example, depending on whether the cathode ray tube 6 is for color display or whether the cathode ray tube 6 is for monochromatic display, or whether the cathode ray tube 6 has a screen size longer in the vertical direction than in the horizontal direction, or whether the cathode ray tube 6 has a screen size longer in the horizontal direction than in the vertical direction, the display memory 5, for use in connection with the particular cathode ray tube 6, may take on a different construction.
Because of the foregoing reason, when an attempt is made to use the electric appliance for office automation for the presentation of both colored display and monochromatic display, an additional cathode ray tube 8 for color display, where the cathode ray tube 6 is used for monochromatic display, or for monochromatic display where the cathode ray tube 6 is used for color display, is required in combination with an additional display memory 7 dedicated to such additional cathode ray tube 8. Moreover, when one combination of cathode ray tube and dedicated display memory is in use, the other combination of cathode ray tube and dedicated display memory is not utilized in operation. Therefore the utilization of one display memory in the electric appliance for office automation cannot be effectively and efficiently maximized. This appears to constitute an obstruction to an attempt to reduce the number of display memories used. Hence, it is an obstruction to an attempt to reduce the manufacturing cost of the electric appliance as a whole.